Fluids, such as oil, gas and water, are commonly sought out and recovered from subterranean formations below the earth's surface using a variety of drilling rigs. These drilling rigs typically drill long, slender well bores into the earth formation to establish a fluid communication between the fluid deposits and the surface through the drilled well bore. During the drilling process logging tools are used to measure the properties of the earth formation along the well bore, such as well bore depth, bulk density, resistivity and porosity. These logging tools are well known and use various techniques to determine the geophysical properties of the earth formation. From these properties, the surrounding formation can be characterized and the information used to determine the likelihood of the presence of hydrocarbons in the formation and/or the ease of producing these hydrocarbons.
For several reasons, information pertaining to the location of the drill bit, such as drill bit depth and Rate of Penetration (ROP) is of particular interest to the study of the geophysical properties of the surrounding earth formation. First, knowledge of drill bit depth is helpful in determining the composition of the strata in which the drill is currently boring. This information can be used by a drill rig operator to determine the weight, speed, and torque to which a drill bit should be adjusted to obtain the optimum drilling performance. The second reason involves the use of the drilling fluid used to maintain control and stability of the borehole by cooling and lubricating the drill head, conveying the drill tailings to the surface and by keeping the hydrostatic pressures in balance. Because the composition of the drilling fluid is typically selected based on strata properties, such as the rock conditions, the borehole size and the borehole length, information about the strata is important in selecting a suitable drilling fluid composition. The third reason involves the Rate of Penetration (ROP) or the rate at which a drill bit penetrates the strata, wherein the ROP provides information about the formation being drilled and the state of the drill bit being used. This information is essential in optimizing the drilling operation. Finally, in directional drilling an accurate estimate of the location is indispensable for adhering to well trajectory and reaching targeted reservoirs in optimal fashion.
Currently, the two most common logging methods being used to determine the depth and other geophysical properties of a borehole are the WireLine method and the Logging While Drilling (LWD) method. The wireline well-logging method employs a well-logging tool, such as a sonde, that is lowered into the well-bore on an electrical cable or wireline. The well-logging tool is an electrically powered measurement device that includes several sensors that measure and collect data regarding the parameters of a borehole and/or its environment. Once measurements have been collected, the measurement data are usually converted into a digital format and transmitted to the surface on the wireline cable. Unfortunately however, although wireline tools are capable of obtaining accurate data, the wireline method is somewhat cumbersome and repetitive in that the wireline cable must be towed along the borehole and that the well must first be drilled before the wireline measurements are conducted and the logs are generated. This is undesirable for several reasons. The first reason involves the time added by having to traverse the borehole multiple times, first to drill the borehole and then to measure the borehole. The second reason is that because the borehole is measured after the borehole has been drilled, the analysis and data collection cannot be conducted on a concurrent basis. Thus, presently information is not available to allow a drill team to direct a drill string in relation to depth, attitude, or inclination using concurrent data analysis.
On the other hand, the LWD method provides for a real-time quantitative analysis of the sub-surface formations during the actual drilling operation and can be run to allow the drill team to better direct the drill string during drilling. The LWD logging method typically includes drilling a borehole into the earth and recording information regarding the geophysical properties of the borehole from sensors, which are typically disposed above the drill bit. The log of these measurements produces a record of various geophysical properties relative to the borehole depth. Unfortunately however, although the LWD method is capable of obtaining data on a real-time basis, the LWD method includes inherent inaccuracies. Further, the current LWD tools do not allow for borehole depth measurements that are independent of a surface tracking system. Because the drill bit does not necessarily move in synchronization with the tail end or surface end of the piping, movement of the drill bit may not be immediately noticed at the surface. As a result, depth measurements made close to the drill bit may be inaccurate. Further, during the drilling process, the drill string typically experiences vibrations and/or rotations which may cause warping in the drill pipe, adding further to the inaccuracies of the LWD measurements.
An additional way to obtain an accurate measurement of the borehole depth is to measure the drill string pipes before sending them down-hole. Because, this measurement is based on what is observed at the surface, the measurements may not accurately translate to the subterranean level due to stretching of the drill string or due to stick or slip. As such, it would be imprudent to have a drilling team rely on measurements taken from observations that cannot be confirmed. Further, because what is observed at the surface may not accurately translate to the subterranean level, it is possible that synchronization problems can occur.